The present invention relates a resin composition comprising a polyamide resin. Particularly, the present invention relates to a flame-retardant polyamide resin composition.
Several methods have been proposed for imparting flame-retardancy to polyamide resin compositions. In general, such method is to mix a flame retardant into polyamides. As the flame retardant used for this purpose, there are known, for example, halogen-based flame retardants, nitrogen-based compounds and metal-containing compounds. In case of using halogen-based flame retardants, however, there is a problem for safety of halogen-containing gases generated on combustion of the compounds. On the ether hand, the metal-containing compounds are unsatisfactory in their flame-retarding effect.
Regarding the nitrogen-based compounds, Japanese Patent Application Laid-Open (KQKAI) Nos. 50-105744 and 53-31759 teach a method for improving flame retardancy of polyamides by adding thereto melamine-, cyanuric acid or other triazine-based compounds such as melamine cyanurate. The method, however, still involved the problem that in case where an inorganic filler, especially a fibrous material such as glass fiber, is blended in polyamides, the filler may behave like a candlewick to adversely affect flame retardancy of polyamides. There has also been the of generating a "plateout" phenomenon in which the flame retardant sublimes in the molding article to deposit on the mold, causing sticking in the cavity or deteriorating of visual appearance of the molded article, or "blooming", i.e. a phenomenon in which part of the flame retardant separates out on the surface of the molded article.
Also, Japanese Patent Application Laid-Open (KOKAI) No. 53-49054 teaches a flame retardant composition in which melamine phosphate is blended. However, by mere addition of a salt of phosphoric acid and melamine, the mixing and extruding workability are rather deteriorated because of poor compatibility with polyamides to impair the visual appearance of the molded article.
Further, Japanese Patent Publication (KOKOKU) No. 61-49342 discloses a flame retardant composition prepared by blending melem or mellon (melem and mellon are calcined products of melamine to polyamide) to polyamide resin. Still further, Japanese Patent Application Laid-Open (KOHYO) No. 10-505875 (WO96/09344) discloses a flame retardant composition prepared by blending a reaction product of melem with phosphoric acid to polyamide. However, the above-mentioned flame retardant compositions are not sufficient in flame retardancy. Especially, in the flame retardant composition of Japanese Patent Application Laid-Open (KOHYO) No. 10-505875, the water absorbance property of the composition becomes high and the dispersibility to the polyamide becomes insufficient because of high water solubility due to the phosphate. As a result, the strength and elasticity thereof may be reduced. Further, the hot bend strength (temperature dependency of bending modulus) thereof may be reduced. Also, when the flame retardant compositions absorb water, reaction product of melem-phosphate is easily transferred to the surface of a molding product thereof, thereby deteriorating the surface appearance.
As a result of the present inventors' earnest studies to solve the above problems, it has been found that by blending (i) a salt of polyphosphoric acid and melam or a melam derivative, (ii) a salt of polyphosphoric acid and melem or a melem derivative, or (iii) mixture thereof with a polyamide resin, a polyamide resin composition has excellent thermal and mechanical properties as well as flame retardancy, and is also capable of providing the molded articles with excellent visual appearance. The present invention has been attained on the basis of the above finding.